The present invention relates to the making of seamless tubes or pipes by means of driven hot rolls which are arranged in various frames, and under utilization of a long, cylindrical mandrel.
The German printed patent application No. 15 27 608 discloses such an arrangement, and it discloses further a method in which the mandrel undergoes particular motions during the rolling process. That motion (or, better, displacement) covers a span the length of which is considerably smaller than the length of the blank as well as the displacement of the blank. Advancing of the mandrel is particularly limited in that the mandrel is introduced in the beginning of rolling in order to extend up to the fourth the fifth or even the sixth rolling stand in a downstream direction of rolling. The mandrel is, thereafter, shifted just sufficiently to ensure full support of the rolled stock during subsequent passes.
The speed of advancement of the mandrel is quite low in the instance above. The mandrel is actually moved by the transmission of motion from the rolls via the stock around the mandrel. In fact, the rolls transmit a considerable tension force upon the mandrel whose forward motion on account of that force must be retarded. The German printed patent application No. 15 27 608 refers specifically to these forces, the transmission of which is due to high friction between the tubular or hollow blank and the mandrel.
Following the completion of rolling, the mandrel rod must be retracted so that the next hollow can be rolled. The retraction of the mandrel and, possibly, an exchange for another one is a part of the rolling cycle and, in some fashion, enters into the overall consideration of power consumption. Moreover, this known method requires that additional steps be taken to avoid breakdown and other interferences. For instance, the mandrel must be supported in between the various rolling stands as long as the rolled stock does not support the mandrel. Also, the flow of cooling water has to be stopped after rolling, up to rolling of the next hollow blank. This is necessary in order to avoid washing off the lubrication layer from that portion of the mandrel that has entered the stands and frames. Turning off the coolant flow after rolling is also required in order to avoid a thermoshock of the freshly exposed mandrel which, at that point, has attained a temperature of approximately 970.degree. Kelvin. Such a thermoshock can readily lead to cracking of the mandrel.
The German printed patent application No. 23 45 056 deals with the problem resulting from relative motion between the blank and the mandrel during rolling. That motion heats the mandrel and wears it out. This particular reference is concerned with the making of seamless tubing in cooperation with a push bench, i.e., a continuous rolling mill with stands that are not driven. The deformation process of the material is different here from the case of a mill having driven stands. The front or leading end of the mandrel rod traverses two stand spacings in the same time span which the front end of the blank requires in order to arrive at the last one of eight stands after the end of the hollow has entered the last roll stand. This will result in an advance of the mandrel rod at a rate which is approximately 15% of the exit speed of the tube and, thus, reduces the wear of the mandrel rod.
In the case of a continuous rolling mill, having driven stands, the rate of progression of the mandrel is lower, e.g., in the order of 5% to 10% of the speed of the tube, assuming that seamless tubes are rolled on a mandrel having a length that is approximately 5% to 10% of the length of the rolled tube. German printed patent application No. 27 01 824 describes this method of working, particularly for improving the quality of the tube and for increasing the life of the mandrel.
Contrary to the earlier described method as per German printed patent application No. 15 27 608, having a vary low-controlled mandrel speed, the continuous method does not stop and hold the mandrel at any time during rolling so that the mandrel speed during rolling adapts to the different conditions during entering and exiting of the stock. This results in a unequal material flow which, in turn, requires additional expenditure in electronic and electric control equipment in order to ensure an adequate quality of the product within prescribed tolerances.
Applying the continuous method to the rolling of longer pipes of larger diameter encounters, as a restriction, an unfavorable ratio of blank length to length of the mandrel rod. The length of the mandrel rod is limited by restraints on manufacturing and is ultimately determined by the value of the force for stripping a thin tube. Free-running mandrels have a length close to the stripping length.